An apparatus for measuring nuclear magnetic resonance comprises essentially an apparatus for creating a static magnetic field (or simply static field) B.sub.O directed along an axis Z'Z, apparatus for creating a first field gradient along the same axis and apparatus for creating a second field gradient along a plane perpendicular to this axis. With such constant fields, a line can be discriminated in a slice of an examination volume. Since the magnetic moments of the nuclei of bodies subjected to these fields are oriented by the static field, precession is obtained by means of an alternating excitation magnetic field. The frequency of this excitation field is determined as a function of the intensity of the static field. The frequency corresponds to the free precession rate of the moments of the nuclei under the effect of this static field. Typically, for a static field varying between 0.15 teslas and 2 teslas this frequency lies between 6 MHz and 80 MHz and therefore the excitation field is often referred to as an r.f. field. For a given structure of the means creating the static field and so for a given intensity thereof, the excitation frequency is determined. The r.f. field is orthogonal to the static field. The field lines of the r.f. field are therefore contained in planes parallel to the direction of the second field gradient.